The invention relates generally to ion implantation and, more particularly, to an apparatus and method for generating oxygen ions.
Ion implantation is a conventional technique for introducing dopants into semiconductor materials. A desired dopant may be ionized to form a plasma in an ion source. The ions are accelerated from the source to form an ion beam of selected energy which is directed at the surface of a wafer. The ions in the beam penetrate into the bulk semiconductor material to form an implanted region.
In some processes, oxygen is implanted into a semiconductor material. Some conventional implantation processes have generated oxygen plasmas by introducing oxygen gas into an arc chamber of the ion source using a gaseous source feed. If the ion source uses an RF or microwave source to generate the plasma, the presence of oxygen gas in the arc chamber may not be problematic. However, if the ion source uses a hot filament formed of an oxidizable metal (e.g., tungsten) to generate the plasma, the presence of oxygen gas in the arc chamber can cause oxidation of the hot filament. This oxidation can significantly reduce filament life and may result in filament metal deposits on the insulators, both inside and outside the arc chamber.
One approach to address the oxidation problem has been to use a less oxidizing source gas, such as CO2 or CO. However, some oxidation of the filament still may occur and carbon deposits may form inside the ion source. Another approach has been to use a very dilute mixture of an oxygen-containing gas (e.g., NO, N2O, and the like) and an inert gas, such as Argon. This approach, however, is generally effective only for low ion dose requirements because of the low oxygen concentration.
It would be desirable to have an ion implantation method and apparatus which can generate an oxygen ion beam from a hot filament source, while limiting oxidation of the filament.
According to the methods and apparatus of the invention, an oxygen ion containing plasma is generated using a hot filament ion source. The oxygen ions in the plasma come from an oxide source (e.g., a metal oxide) which has a lower free energy of formation than that of the filament metal oxide (e.g., WO3) at the operating temperatures of the ion source. Consequently, oxidation of the filament and other metal components of the arc chamber is limited, or even prevented. Thus, the methods and apparatus of the invention can advantageously lead to longer filament lives as compared to certain conventional processes that generate oxygen plasmas using hot filament sources.
In one aspect, the invention provides an ion source comprising a chamber, a metal filament housed within the chamber, and an oxygen source component housed within the chamber. The oxygen source component comprises an oxide compound which has a lower free energy of formation than the oxide of the filament metal over the operating temperature range of the ion source.
In another aspect, the invention provides an ion source. The ion source comprises a chamber, a metal filament housed within the chamber, and an oxygen source component housed within the chamber. The oxygen source component comprises a metal oxide of the group consisting of Ga2O3, B2O3, SiO2, Al2O3, TiO2, ZrO2, and MgO. The ion source further comprises a source gas supply connected to the chamber.
In another aspect, the invention provides a repeller unit for an arc chamber of an ion source. The repeller unit comprises a conductor plate and a connector connecting the conductor plate to a voltage source. The repeller unit further comprises an ion source component mounted on the conductor plate and exposed to the interior of the arc chamber when the repeller unit is mounted therein. The ion source component comprises a solid oxide compound which is a member of the group consisting of Ga2O3, B2O3, SiO2, Al2O3, TiO2, ZrO2, and MgO.
In another aspect, the invention provides a method of generating oxygen ions in an arc chamber of an ion source having a metal filament therein. The method comprises the step of providing a solid oxygen source within the arc chamber. The solid oxygen source comprises an oxide compound which has a lower free energy of formation than the oxide of the filament metal over the operating temperature range of the arc chamber. The method further comprises the steps of feeding a source gas into the chamber, passing current through the filament, and causing electrons to be emitted from the filament. The electrons collide with source gas molecules to create fluoride ions that chemically interact with the solid oxygen source to form oxygen ions.
In another aspect, the invention provides a method of generating an ion beam of a desired species, selected from the group consisting of Al, Ga, B, Si, Ti, Zr, and Mg ions, in an ion implanter apparatus comprising an ion source including an arc chamber having an electron emitting filament therein of an oxidizable metal. The method comprises the steps of providing a solid oxide of the desired species in the arc chamber; feeding a source gas into the chamber; generating an arc in the arc chamber by thermionic electron emission from the filament; extracting an ion beam comprising ions of the desired species from the arc chamber; and separating the desired species ions from other components of the ion beam to form the ion beam of the desired species.
Other aspects, features and advantages will become apparent from the following detailed description and drawings when considered in conjunction with the claims.